This invention relates to an analog pulsed heat trace monitor or the like, and more particularly, to a system using analog pulsed signals to monitor a condition or changing condition which can be expressed by a whole digit or other alphanumeric symbol (such as temperature, pressure or flow). This invention is disclosed in the context of a system for monitoring the condition of systems protected by elongated heaters of the kind generally known as "heat tracing" or "heat trace". In such an application, the invention provides a system whereby the power which drives the heat tracing is used to also drive a signal generator, the output of which is applied to the heat tracing and sensed by a remote receiver. Because the signal is a pulsed one, impressed upon the power supply for the heat tracing, and because certain components of the system are maintained at earth ground, interference and "cross-talk" between heat tracing circuits is eliminated.
The method at present most commonly used to monitor the condition of a heat traced piping systems is to measure the temperature at the end of each heat traced circuit. In the case of a heat-trace based freeze protection system, the heat trace is expected to keep the temperature of the protected substrate (such as a pipe) at 40.degree. F. or higher when the ambient air temperature is at or below 40.degree. F. Conventionally, the temperature of the substrate is detected by a temperature sensing transducer associated with the substrate. Conventional transducers may be in the form of a thermocouple or resistance temperature detector (RTD), and the output signal of the sensor is, in conventional practice, brought to a receiver by a separate conductor provided for that purpose.
A control panel, with which a power supply may be associated, conventionally provides a circuit breaker for each heat trace circuit, and an annunciator panel typically providing an indicator light for each circuit. The thermocouples or RTDs are wired to the indicating windows, and typically, by depressing a "test" button adjacent to a particular window, a digital readout of the sensed temperature of the heat traced substrate assigned to the window can be had.
In the event of a circuit failure, causing the sensed temperature of the substrate to fall below an alarm limit, the window assigned to that circuit typically flashes red, and an audible alarm will sound.
A shortcoming of such arrangements is that failures of many types can trigger a low temperature alarm, but there is no way to know which component of the entire circuit has failed; all failure modes in such systems are temperature-related.
Another objection to prior art systems has been the cost of installing, along the entire length of the heat traced substrate, additional wiring for the monitoring system (i.e. RTD or thermocouple wire). Because heat trace circuits can be as long as 500 ft., and the distance from the power connections for the apparatus to the control panel on the order of 100 ft., extensive wiring can be involved, and the cost of such wiring can be as much as 20% of the total installation cost.